The present invention relates to an electric power steering system for a motor vehicle, and more particularly to a system for controlling a return torque in accordance with driving conditions of the vehicle.
U.S. Pat. No. 4,664,211 (Japanese Patent Laid Open No. 61-132465) discloses an electric power steering system. The system is provided for producing an assist signal dependent on the torsion torque signal from a torsion torque sensor and on the vehicle speed, an auxiliary assist signal dependent on the differentiation of the torsion torque signal, a return torque signal depending on a steering angle signal from a steering angle sensor, for returning the steering wheel to a neutral position, and a damping signal depending on the differentiation of the steering angle signal. These signals are added for controlling an electric motor.
In the steering system, the return signal is determined only by the steering angle signal. Accordingly, if the return signal is set to provide a sufficient return torque when the lateral acceleration exerted on the motor vehicle during cornering is in a low range, when the lateral acceleration is high during cornering at a low or middle vehicle speed, the return starting speed of the steering wheel becomes insufficient, resulting in delay of the returning of the steering wheel. In order to eliminate such a disadvantage, the applicant proposed a system for correcting the return torque in accordance with the lateral acceleration.
However, the system is arranged to correct the return torque by a correcting value only dependent on the lateral acceleration. At cornering at a high vehicle speed, the lateral acceleration becomes high, so that a large correcting value is added to the return torque in spite of a small steering angle. As a result, the return torque becomes excessively large.